Mineral oil based lubricants are conventionally produced by a separative sequence carried out in the petroleum refinery which comprises fractionation of a paraffinic crude oil under atmospheric pressure followed by fractionation under vacuum to produce distillate fractions (neutral oils) and a residual fraction which, after deasphalting and solvent treatment may also be used as a lubricant basestock usually referred to as bright stock.
Neutral oils, after solvent extraction to remove low viscosity index (V.I.) components are conventionally subjected to dewaxing, either by solvent or catalytic dewaxing processes, to the desired pour point, after which the dewaxed lubestock may be hydrofinished to improve stability and remove color bodies. This conventional technique relies upon the selection and use of crude stocks, usually of a paraffinic character, to produce the desired lube fractions of the desired qualities in adequate amounts.
The range of permissible crude sources may, however, be extended by the lube hydrocracking process which is capable of utilizing crude stocks of marginal or poor quality, usually with a higher aromatic content than the best paraffinic crudes. The lube hydrocracking process, which is well established in the petroleum refining industry, generally comprises an initial hydrocracking step carried out under high pressure in the presence of an amorphous bifunctional catalyst which effects partial saturation and ring opening of the aromatic components which are present in the feed.
The hydrocracked product is then subjected to dewaxing in order to reach the target pour point since the products from the initial hydrocracking step which are paraffinic in character include components with a relatively high pour point which need to be removed in the dewaxing step.
In theory, as well as in practise, lubricants should be highly paraffinic in nature since paraffins possess the desirable combination of low volatility and high viscosity index.
Conventional hydrocracking catalysts combine an acidic function and a hydrogenation function and are considered bifunctional. The acidic function in the catalyst is provided by a porous solid carrier such as alumina or silica-alumina. Amorphous materials have significant advantages for processing very high boiling feeds which contain significant quantities of bulky polycyclic materials (aromatics as well as polynaphthenes) since the amorphous materials usually possesses pores extending over a wide range of sizes and the larger pores, frequently in the size range of 100 to 400 Angstroms (.ANG.) are large enough to provide entry of the bulky components of the feed into the interior structure of the material where the acid-catalyzed reactions may take place.
Crystalline materials, especially the large pore size zeolites such as zeolites X and Y, have been found to be useful for a number of hydrocracking applications since they have the advantage, as compared to the amorphous materials, of possessing a greater degree of activity, which enables the hydrocracking to be carried out at lower temperatures at which the accompanying hydrogenation reactions are thermodynamically favored. In addition, the crystalline catalysts tend to be more stable in operation than the amorphous materials such as alumina. The crystalline materials may, however, not be suitable for all applications since even the largest pore sizes in these materials, typically about 7.4 .ANG. in the X and Y zeolites, are too small to permit access by various bulky species in the feed. For this reason, hydrocracking of residual fractions and high boiling feeds has generally required an amorphous catalyst of rather lower activity.
The bifunctional catalyst also comprises a metal component which provides the hydrogenation/dehydrogenation functionality. The metal component typically comprises a combination of metals from Groups IVA, VIA and VIIIA of the Periodic Table (IUPAC Table) although single metals may also be encountered. Combinations of metals from Groups VIA and VIIIA are especially preferred, such as nickel-molybdenum, cobalt-molybdenum, nickel-tungsten, cobalt-nickel-molybdenum and nickel-tungsten-titanium. Noble metals of Group VIIIA especially platinum or palladium may be encountered but are not typically used for treating high boiling feeds which tend to contain significant quantities of heteroatoms which function as poisons for these metals.
In U.S. Pat. No. 5,128,024 zeolite beta is described as a hydrocracking catalyst for heavy hydrocarbon oils. It can be contrasted with the conventional hydrocracking catalysts because it demonstrates ability to attack paraffins in the feed in preference to the aromatics. This reduces the paraffin content of an unconverted hydrocracker effluent fraction thereby lowering the pour point of the product.
However, the aromatics as well as other polycyclic materials are less readily attacked by zeolite beta than the paraffinic materials with the result that they pass through the process and remain in the product, consequently reducing the product V.I. While zeolite beta-catalyzed processes have shown effectiveness for dealing with highly paraffinic feeds, the high isomerization selectivity of the zeolite beta catalysts, coupled with a lesser capability to remove low quality aromatic components, tended to limit the application of the process to feeds which contained relatively low quantities of aromatics.
In contrast to zeolite beta, amorphous catalysts used in lube hydrocracking are relatively non-selective for paraffin isomerization in the presence of polycyclic components but have a high activity for cracking resulting in a low overall yield and high dewaxing demands. While, as mentioned above, the zeolite beta-catalyzed processes are capable of achieving higher yields, since the zeolite has a much higher selectivity for paraffin isomerization, the aromatics are not always effectively dealt with in lower quality feeds.
In addition to the lube hydrocracking processes described above, hydrocracking has been employed in the production of fuels such as gasoline and middle distillates.
Fuels hydrocracking is a process which has achieved widespread use in petroleum refining for converting various petroleum fractions to lighter and more valuable products, especially distillates such as jet fuels, diesel oils and heating oils. Hydrocracking is generally carried out in conjunction with an initial hydrotreating step in which the heteroatom-containing impurities in the feed are hydrogenated without a significant degree of bulk conversion. During this initial step, the heteroatoms, principally nitrogen and sulfur, are converted to inorganic form (ammonia, hydrogen-sulfide) and these gases may be removed prior to the subsequent hydrocracking step although the two stages may be combined in cascade without interstage separation as, for example, in the Unicracking-JHC process and in the moderate pressure hydrocracking process described in U.S. Pat. No. 4,435,275.
In the second stage of the operation, the hydrotreated feedstock is contacted with a bifunctional catalyst which possesses both acidic and hydrogenation/dehydrogenation functionality. In this step, the characteristic hydrocracking reactions occur in the presence of the catalyst. Polycyclic aromatics in the feed are hydrogenated, and ring opening of aromatic and naphthenic rings takes place together with dealkylation. Further hydrogenation may take place upon opening of the aromatic rings. Depending upon the severity of the reaction conditions, the polycyclic aromatics in the feed will be hydrocracked to paraffinic materials or, under less severe conditions, to monocyclic aromatics as well as paraffins. Naphthenic and aromatic rings may be present in the product, for example, as substituted naphthenes and substituted polycyclic aromatics in the higher boiling products, i.e. the hydrocracker bottoms, depending upon the degree of operational severity.
Under the conditions of fuels hydrocracking, the crystalline hydrocracking catalysts generally tend to produce significant quantities of gasoline boiling range materials (approximately 330.degree. F.-, 165.degree. C.-) as product. Since hydrocracked gasolines tend to be of relatively low octane and require further treatment as by reforming before the product can be blended into the refinery gasoline pool, hydrocracking is usually not an attractive route for the production of gasoline. On the other hand, it is favorable to the production of distillate fractions, especially jet fuels, heating oils and diesel fuels since the hydrocracking process reduces the heteroatom impurities characteristically present in these fractions to the low level desirable for these products.
The selectivity of crystalline aluminosilicate catalysts for distillate production may be improved by the use of highly siliceous zeolites, for example, the zeolites possessing a silica:alumina ratio of 50:1 or more as described in U.S. Pat. No. 4,820,402 (Partridge et. al.).
In conventional hydrocracking processes for producing middle distillates, especially jet fuels, from aromatic refinery streams such as catalytic cracking cycle oils, it has generally been necessary to saturate the aromatics present in the feed to promote cracking and to ensure that a predominantly paraffinic/naphthenic product is obtained. The hydrocracked bottoms fraction is usually recycled to extinction or blended with the distillate product even though it is highly paraffinic, because of the aromatic-selective character of the catalyst, and could form the basis for a paraffinic lube stock of higher value than the distillate produced by cracking it.
U.S. Pat. No. 4,851,109 describes integration of jet fuel and middle distillate production by moderate pressure hydrocracking and catalytic lube production using isomerization dewaxing over a catalyst based on zeolite beta. This process minimizes hydrogen consumption while producing naphthas and middle distillates of high quality. The bottoms fraction from moderate pressure fuels hydrocracking is processed in a hydroisomerization/hydrocracking step to produce a distillate fraction or an aromatics-rich fraction which may be used to produce a premium grade lube base stock using conventional processing technology. Even though an improvement in lube production is offered by the use of zeolite beta in a second stage hydroisomerization/hydrocracking step, the aromatics, and especially the polycyclic aromatics, still pose a challenge.